We live in a highly segregated society. Not just racially, of course. We are segregated by class, religion, culture, lifestyle, age…etc. We tend to hang out with people like ourselves AND hang out in places that cater to people like ourselves. Whenever I see a couple that are obviously different from each other, I’m always curious how did they cross each other’s path.
Otherwise, no idea. My guess would be that one or both of them was attracted that way and was outgoing enough to take the long shot when an opening came.
They work together or go to class together. It’s a good way to get to know someone in a non-threatening environment. Plus you can get to know who they are over a long period and may find their personality is attractive to you.
I have never really met anyone like myself, so I don’t know that your premise is true at all.
I don’t think the original premise holds up in cities.
Just take a look at any area in New York or London - full of diversity.
Many opposites aren’t as completely opposite as they look from outside. My brother and his wife make lots of people say that “opposites attract,” but they met through an interparish youth group. She never took part into any of the group’s activities again until they got married, he’d been involved with the group for years and has never stopped; he’s a lot more churchgoing than she is (she’s one of those people who have serious problems changing activities, it’s like the idea of all the things she has to do in order to go somewhere is overwhelming) but they wouldn’t have met if she’d been an atheist.
I met my wife at a fundraiser for the local public library. I was a longtime donor, and she was a volunteer for the event. Otherwise there’s no way we ever would have meant. Our interests, and thus our circles, were way too different.
I’ve dated women who waited on me in restaurants. No PhD, no house in the country and another in town, but they might like some of the same things like going to art showings.
There’s always a limit to how “opposite” you are. I mean, I’m living. Does that mean my opposite has to be dead?
Once you get that out of the way, you just realize that you have to have had something in common, and that’s how you meet. Even if that something in common is being in the same place at the same time.
I have to remember that line. 
I met my “opposite” in college. Where it seems like you’re most able to meet such people.
Craigslist. 
Yeah, but then there are still lots of similarities, like you’re both into having sex with balloons while wearing fursuits and listening to Michael Bolton.
I met my ex in grad school and we were very different people. In fact, I think the only real thing we had in common was that we were both studying Computer Science, but even in there, we were studying very different things (I’m more interested in algorithms, datamining, and that sort while she was into stuff like GUIs). We had different religious views, politics, ethnic and cultural backgrounds, outlooks on life, musical tastes, interests, etc.; you name it, and it was different.
Really, I don’t see why it would be hard to meet opposites. For instance, two people may work together, but it doesn’t mean anything else they have is in common. Even more, if you are into the bar scene, or some other similar way of pretty much meeting random people, you don’t know anything about that person when you approach them so you’re probably just as likely to meet someone who is very different as you are someone who is very similar.
So, really, I guess it all comes down to how you meet them. If it’s through a friend, or a place where people tend to have similar views like church, then you’re probably more likely to be similar; otherwise, I imagine it’s basically a crap shoot.
My husband and I used to be a lot more “opposite” than we are now, after 15 years of marriage. But we met because of something we had in common. We both knew my mom. ::shrug::
My parents seemed to be the same, both immigrants from Yugoslavia, both highly educated, (doctorate degrees), but once you got to know them, you quickly saw they were really opposites.
For example both were creative, but my dad was really the creative type that was “lost in the clouds” while my mother was creative but “planted in reality.” So one might think they were both creative and while this was true superficially, in reality they were opposites. My father would get lost in his creativity, while my mum could say “OK enough of this, time to go for the real world.”
This meant they worked well as where one had a major weakness the other filled in with strength. So in their case it worked well.
For instance, my dad couldn’t keep a dime in his pocket, my mother planned out each penny. This worked 'cause my father was laid back and didn’t mind turning his paycheck over to mum saying “OK give me my allowance.”